The present invention is directed generally to a testing apparatus and method for an ISDN (Integrated Services Digital Network) system, and, more particularly, to a testing apparatus and method that can be used at a terminal equipment (TE) location to determine whether a satisfactory connection is available at the TE location from a network terminator (NT), and to identify the source of the problem if a satisfactory connection is not provided.
Recently, extensive work has been done in developing ISDN systems which are expected to greatly improve services available to subscribers. ISDN is presently being promoted by telephone companies in an effort to improve quality, capacity, and variety of services. Preliminary ISDN specifications have been introduced by CCITT, the international communications standards committee. In the U. S. and Canada, subsets and minor variations of these standards are being defined by the major central office equipment vendors, such as AT&T and Northern Telecom.
The technical concept of ISDN is being accomplished by using the existing copper telephone lines to carry a digital signal instead of the usual analog voice (limited to 3000 Hz). The digital signal is transmitted at a rate of 144,000 bits per second (bps), offering two 64,000 bps channels (known as "B" channels) for voice or data calls, and one 16,000 bps channel (identified as the "D" channel) for establishing and maintaining the call connection to the network (and also used for sending data packets using the X.25 protocol) as a basic service. Implementation of ISDN requires new central office switching and transmission equipment, and sometimes dramatically different terminal equipment (e.g., telephones, modems, data terminals) at the user or subscriber end of the telephone line. At present, ISDN is starting limited technical field trials in several cities around the U. S., and is already being widely implemented in Europe.
Technicians and installers of ISDN circuits often need to identify the source of trouble in the event of problems in making or answering a voice or data call. As noted above, in basic ISDN service, the voice or data information is carried by one of two B channels on the ISDN circuit. The call transactions to make a connection, or link, are established via the D channel. Messages sent to and from the central office on the D channel contain the information defining the status of the link. It is very important to know what part of the link activity is failing to determine if the trouble is in the local equipment or elsewhere. All D channel information may be displayed in the following formats:
1. Binary. PA1 2. Hex. PA1 3. Bus activation state. PA1 4. OSI layer 1 HDLC frames. PA1 5. OSI layer 2 LAPD/LAPD decoder frames. PA1 6. OSI layer 3 Q.931 and decoder X.25 frames. PA1 Line transmission termination. PA1 Line maintenance and performance monitoring. PA1 Timing. PA1 Power transfer, extracting power from the line to drive at least the "wake-up" portion of the terminal. PA1 Parts of the multiplexing functions. PA1 Termination of the T interface which may include multi-drop termination and associated contention resolution functions. PA1 Protocol handling or handling that part of the protocol associated with information transfer across a network. PA1 The higher-level parts of the multiplexing function. PA1 Switching and concentration functions. PA1 Maintenance functions. PA1 Interface functions to the S and T interfaces. PA1 Reference point T (T for Terminal) Separates the network provider's equipment from the user equipment. Provides a standardized interface between equipment, sending and receiving, validating and timing information to the network and to terminal equipment devoted to the use of this information. PA1 Reference point S (S for System) Separates the user terminal equipment from the network functions of the terminal. PA1 Reference point R (R for Rate) Provides a non-ISDN interface between non-ISDN compatible user equipment and adapter equipment. Such an interface may well comply with one of the CCITT X series interface recommendations. PA1 Reference point U (U for User) Interface between NT1 and the transmission line.
FIG. 1 shows a basic ISDN terminal which the present invention can be used in conjunction with. The terminal is divided into functional groupings. By separating functions conceptually in this way, it is possible to identify and specify the interfaces between them. A description of the functional groups is as follows, noting that this description is confined to the functions of basic access.
NT1--Network Termination 1. This is the group of functions which terminate the transmission line. As such, it is seen in the CCITT recommendations as belonging to the network provided, i.e., the owner of the transmission line. The functions of the NT1 are described as: PA0 NT2--Network Termination 2. This is the group of functions which give the terminal its particular "character". An NT2 could be a PABX if access is primary, a LAN or a terminal controller. The functions of the NT2 are described as: PA0 The NT2 functional group may be more or less complex depending on the application. The range extends from the quite complex function of a PABX down to relatively simple functions required for a time division multiplexer. In specific, simple cases all the functions may be adequately performed by NT1, and NT2 becomes merely physical connections. PA0 TE--Terminal Equipment. This is the device itself. It could be a digital telephone, a CADCAM workstation, a computer terminal, etc. PA0 TE1--Terminal Equipment Type 1. Such terminal equipment complies with ISDN user-network interface recommendations and therefore supports interface S. PA0 TE2--Terminal Equipment Type 2. Such a terminal supports the same functions but does not comply with the ISDN user-network interface recommendations. It must therefore interface with the ISDN access via a PA0 TA Terminal Adapter which converts the non-ISDN interface functions into ISDN acceptable form at reference points S or T.
This description has largely revealed the nature of the interfaces. A further description is as follows:
The terminology from FIG. 1 will be used throughout the rest of the specification. This terminology is presently well accepted standard terminology, as discussed, for example, in the textbook, "The Integrated Services Digital Network: from Concept to Application", by John Ronayne, published by John Wiley & Sons, Inc., 1988, which is hereby incorporated by reference. Applicants also hereby incorporate by reference related application Ser. No. 192,174 and application Ser. No. 192,175 by S. Coffelt et al, filed on May 10, 1988. It is to be noted that throughout the specification, the term "TE" (terminal equipment) will be used to generically identify both the use of ISDN compatible equipment TE1 and ISDN non-compatible equipment TE2 together with a commercially available terminal adapter (TA).
It is also to be noted that throughout the specification the term "NT" (network termination) will be used to generically identify both the NT1 and NT2 shown in FIG. 1, and that the term "S/T interface" will be used generically to cover both the S interface and the T interface referred to in FIG. 1. This is done since it is possible for the present invention to be used both at the S interface and the T interface of FIG. 1 depending upon the particular system being used. This, of course, is a reflection of the particular NT1 and NT2 equipment which is being used.
In the past, testing systems for ISDN systems have been quite complex. Specifically, prior test equipment has provided very detailed information regarding all of the protocols of the ISDN system. Although this is, of course, quite useful, it requires a great deal of training for technicians and installers to be able to properly interpret this detailed information.
As a result, it has been difficult in the past for many technicians and installers who do not have the benefit of extensive training to run tests at a TE location to determine if a satisfactory connection is provided at the S/T interface from the NT, and, if not, what the source of the problem is. Accordingly, there is a need for a "user friendly" testing device and method to allow technicians and installers to readily inspect and trouble-shoot ISDN systems at TE locations.